Karachi was hit by a spate of sectarian violence in February
At least 20 people have been killed in violence in the Pakistani city of Karachi, police say.
They say most of the dead were victims of drive-by shootings carried out by unidentified motorcyclists.
The bulk of Wednesday's violence took place between rival ethnic groups in western and eastern parts of the city.
Correspondents say that while Karachi has not been spared Islamist militant violence in recent months, a bigger worry is factional violence.
The city was wracked by clashes between rival ethnic-based political factions for much of the 1990s in addition to sectarian violence between Shias and Sunnis.
The provincial government closed all educational institutions in the city on Thursday and school exams were suspended for the day.
Many of the killings were followed by intense gunfire, police say.
Karachi city police chief Waseem Ahmed said that the dead included at least one member of the Muttahida Quami Movement (MQM) party and four members of Awami National Party (ANP).
The MQM is supported by Karachi's majority Urdu-speaking population whose ancestors migrated from India at the time of Indian partition in 1947. They mostly live in the central parts of the metropolis.
The ANP derives support from the city's ethnic Pashtun population, which is spread across its western and eastern parts.
The BBC's M Ilyas Khan in Islamabad says that the two parties have accused each other of carrying out targeted killings since 2007.
A number of ANP workers were killed in May 2007 when the MQM allegedly resorted to violence to prevent Chief Justice Iftikhar Chaudhry - who at the time had been suspended by military ruler Pervez Musharraf - from addressing lawyers in Karachi. President Musharraf was supported by the MQM.
ANP Sindh province head Shahi Said said that Wednesday's killings were part of an "ongoing process of ethnic cleansing" of Pashtuns in Karachi.
He said the killings were perpetrated by the "same people who were responsible for the 12 May (2007) killings" - a clear reference to the MQM.
But in an official statement on Wednesday night, the MQM blamed the killings on "infighting" between the ANP and those campaigning for a separate Hazara province in the north of the country.
The ANP governs what was known as North West Frontier Province but which last month changed its name to Khyber Pakhtunkhwa.
The party was in the forefront of the campaign for the name change, which was vigorously resisted by people in the Hazara region.
Our correspondent says that people from both regions have a considerable presence in Karachi.